It is desired to measure properties of gas in a flow that may be expected to entrain particles or droplets, such as in flight, where water droplets and/or ice may be entrained in air, and where measurements of total air temperature, relative humidity, pressure, mass flow rate, etc. are desired. Operating aircraft in icing conditions brings some risks. Conditions favourable to ice accretion can be critical for determining safe operating conditions of an aircraft. Equipping aircraft to safely operate in icing conditions may be expensive, and, given a want for accurate prediction of local airspace conditions, a safety margin for aircraft unequipped for flight in icing conditions, may lead to less effective utilization of the aircraft. Moreover aircraft equipped for such flight may need to detect icing conditions for safe operation. Currently no commercially available probes are known to reliably provide both total air temperature (herein TAT) and humidity, even though hygrometers exist, and TAT sensors are known using heated probes (where the heating is controlled to avoid contaminating the TAT measurement, and avoid icing).
The ice crystal and mixed phase icing environments are particularly problematic because substantial heat is required to avoid anti-icing, and this heat risks contaminating the TAT measurements and airborne liquid water content that can contaminate the humidity measurements.
A variety of sensor designs have been proposed. For example, U.S. Pat. No. 8,182,140 to Severson teaches a flow housing on an aircraft skin that has a hollow strut, and a foreaft facing flow tube mounted onto the strut. Free stream air flows through the flow tube and is controlled by an opening at the aft end. A branch flow channel is provided in the strut, and various sensor arrangements are shown.
A similar design is disclosed in U.S. Pat. No. 6,622,556 to May, housing a sample chamber for receiving a first flow diverted from the primary flow path, and an ancillary chamber adjacent to the sample chamber for receiving a second flow diverted from the primary flow path. May teaches a serpentine path through the sensor air flow paths and an aspirator for controlling air flow through the flow chamber.
U.S. Pat. No. 7,370,526 to Ice shows a similar skin-mounted sensor, showing expressly where a total air temperature TAT sensor, and a humidity sensor (inter alia), are to be located. In one embodiment, a flow controlled pressure source (e.g. a fan or pump) is used to control aspiration of a secondary chamber which houses a sensor.
As will be noted in relation to each of these teachings, one important characteristic of these sensors is the ability to preclude ingestion of water droplets and ice from the free stream air flow into the diverted air flows presented to the sensors. Presence of water or ice typically impairs the sensor. This is a considerable problem with the common design of each of these.
An equal problem is encountered in wind tunnels equipped for producing extreme mixed phase icing environments. Researchers have worked on solving the same problem in this context. A variety of embodiments have been tried in this context too. For example, a publication 2011-38-0036 of SAE International published Jun. 13, 2011, entitled Naturally Aspirating Isokinetic Total Water Content Probe: Wind Tunnel Test Results and Design Modifications, to Applicant's Craig Davison, Thomas Ratvasky of NASA, and Lyle Lilie of Science Engineering Associates, shows a design with three sensors in a wind tunnel. FIG. 22 specifically shows a Background Humidity Sampling Probe, an isokinetic IKP and a hot wire probe HWP. The Background Humidity Sampling Probe has a shape of a thickened pipe end, with a gradual thickening of the pipe wall towards the end, which faces downstream.
Orienting the probe inlet opposite to the flow does not accomplish the desired separation, at least in some flow regimes (see p. 4):                “In some cases the measured humidity level by the LiCor implied greater than 100% relative humidity at the spray bars. This was assumed to be caused by the relatively warm water and air injected through the spray nozzles providing the energy to evaporate more water than would be possible if everything was at ambient conditions. This occurred more often at lower velocities and higher LWC levels which would maximize the energy available and minimize the cooling effect, maximizing the available energy from the water for evaporation. Fortunately, as this phenomenon occurred more often at higher LWC levels the effect on the final TWO result was usually small so if the assumptions presented are incorrect the error was minimal.”        
An aspect not mentioned here is that, high liquid water content (LWC) and low velocity (i.e. where the worst contamination was observed) represents the worst condition for entrainment of droplets. At low velocity, due to lower momentum of the droplets, they are more easily drawn in by the sample airflow and the low drag that would remove droplets from the inlet edge of the probe. These probe inadequacies would provide the results that were observed where the humidity was overestimated, i.e. >100%.
More directly, an excerpt from p. 5 states:                “In some cases the background humidity probe appears to have ingested a large quantity of liquid water. An example is shown near the 70 second point in FIG. 5. The LiCor reading increases rapidly and the TDL and Vaisala TWO readings show a corresponding drop. The raw TDL reading, however, shows no corresponding change. If it was truly an increase in background humidity we would expect to see an increase in the raw TDL reading and a resulting constant value for the TDL TWO. It is not until 220 seconds, 150 s after the initial ingestion, that the liquid water in the LiCor tubing appears to have fully cleared.”        
A solution is needed to improve the separation of droplets and ice entrained in a free stream air flow, or like particulate laden gas stream, and provides a particulate free flow for sampling the free stream air flow.